The present invention pertains to multilayer meltblown fibrous webs, methods of manufacturing such webs, and apparatus for manufacturing multilayer meltblown fibrous webs.
The manufacture of meltblown fibrous webs has been discussed in many references, including, Wente, Van A., Superfine Thermoplastic Fibers, 48 Industrial Eng. and Chem. 1342-46 (1956); Report No. 4364 of the Naval Research Laboratories, published May 25, 1954, entitled Manufacture of Superfine Organic Fibers, by Wente, V. A., Boone, C. D., and Fluharty, E. L.; and U.S. Pat. No. 3,971,373 to Braun.
In making meltblown fibrous webs, a thermoplastic polymer or resin is commonly extruded through a row of small, side-by-side orifices into a high velocity gaseous stream that attenuates the emerging material into fibers. The gaseous stream creates turbulence that randomly entangles the fibers to form a coherent nonwoven web on a collector. The collector may be a moving flat belt or rotating cylindrical screen or drum. The resulting nonwoven web is transferred from the collector to a temporary storage roll.
Known processes have a couple drawbacks, namely, they can produce significant waste as a process by-product and they can produce non-uniformities across the web.
Waste (also referred to as weed) is commonly produced at the web edges when manufacturing meltblown fibrous webs. The waste or weed results because the web edges are typically xe2x80x9cfeatheredxe2x80x9d, meaning the edges taper off and do not have the same weight and density as the central portion of the web. The feathering stems from fiber dispersal at the web edges. To eliminate this variation in weight and density, the web edges typically are trimmed off and then discarded as waste, while the central portion of the web is retained for further processing. The wasted material adds to processing costs, especially when in-line web processing is desired.
Known meltblown fibrous webs are typically mono-layer webs that, by definition, have only a single layer. Mono-layer meltblown fibrous webs often suffer from non-uniformities over their cross-web dimension due, for example, to variations in orifice diameter. The variations in orifice diameter can cause non-uniform fiber deposition that, in turn, causes variations in the basis weight in the cross-web dimension. The basis weight is the weight per unit area of the mono-layer web, and it is commonly adjusted by varying the polymer extrusion rate or the collector speed or both. For example, if a higher basis weight web is desired, the collector speed can be reduced and/or the extrusion rate can be increased. Conversely, if a lower basis weight web is desired, the collector speed can be increased and/or the extrusion rate can be decreased.
One approach to overcoming variations in basis weight include laminating multiple webs together using agents such as adhesives or resins and/or by physical processing such as welding. The variations in the multiple webs then preferably average out the non-uniformities such that the minimum acceptable basis weight is achieved over the entire laminated web. One disadvantage to this approach is that some areas of the web can have an excessive basis weight and hence unnecessary amounts of web material. The unnecessary material, as well as the laminating agents and/or processing needed to laminate the webs to form the multi-web products adds to production costs and increases complexity. Furthermore, the agents and/or welds used to laminate the layers can adversely affect the resulting articles"" conformability and flexibility.
Attempts to employ tubular fibrous web processes to achieve a flat web have typically involved forming the tubular meltblown fibrous web and compressing the tube to obtain a flat web without feathered edges. Alternatively, the tubular web may be slit longitudinally so that the tube is opened, thereby producing a flat web with two machine-cut edges. Two such approaches are described in U.S. Pat. Nos. 3,909,174 (Blair et al.) and 4,032,688 (Pall). A disadvantage of these processes is that variations in web thickness may often be helical in nature. As a result, slitting the web longitudinally often causes banded variations in the web density, which variations are located at an angle, commonly referred to as a xe2x80x9cbias angle,xe2x80x9d with respect to the web centerline.
The present invention is directed to overcoming the noted drawbacks in known methods for making meltblown fibrous webs. In one aspect, the present invention provides a new apparatus for manufacturing a meltblown fibrous web. The new apparatus includes (i) a collector that has a generally cylindrical forming surface and (ii) a source that is capable of directing meltblown fibers at the forming surface. The generally cylindrical forming surface can rotate about a longitudinal axis and can simultaneously move parallel to the longitudinal axis, such that a selected point on the forming surface can move in a helical pattern about and along the longitudinal axis from a first end of the collector to a second end of the collector. The helical pattern defines a helix angle relative to the longitudinal axis. The apparatus also includes (iii) a separator that can separate a tubular meltblown fibrous web formed on the forming surface in a direction generally parallel to the helix angle. The separator thus converts the tubular meltblown fibrous web into a non-tubular or flat meltblown fibrous web.
In a second aspect, the present invention provides a method of manufacturing a meltblown fibrous web using a collector having a generally cylindrical forming surface. The forming surface is rotated about a longitudinal axis and simultaneously moves longitudinally in the direction of the longitudinal axis such that a selected point on the forming surface moves in a helical pattern about and along the longitudinal axis from a to first end of the collector to a second end of the collector. The helical pattern defines a helix angle relative to the longitudinal axis. Meltblown fibers are directed at the forming surface as the forming surface rotates and moves longitudinally, such that a tubular meltblown fibrous web is formed on the forming surface. The tubular meltblown fibrous web is then separated along a direction generally parallel to the helix angle to convert the tubular meltblown fibrous web into a non-tubular or flat meltblown fibrous web.
In a third aspect, the present invention provides a multilayer meltblown fibrous web that has a plurality of interconnected layers that contain meltblown fibers. At least one of the fiber-containing layers has a feathered edge. The web also has two separated edges. The feathered edge is located between the separated edges, and the separated edges and the feathered edge are generally parallel to each other. The multilayer meltblown fibrous web may be used in a variety of articles such as filters for masks or respirators.
The multilayer meltblown fibrous webs of the present invention are produced on a collector having a forming surface in the general shape of a cylinder where the forming surface rotates about the longitudinal axis of the cylinder. While the forming surface rotates as such, it is simultaneously advanced parallel to and along the longitudinal axis. As a result, any particular point on the forming surface moves along a helical path during web manufacture.
A meltblown fiber source is directed at the forming surface along at least a portion of the longitudinal length of the collector, thereby forming a layer of meltblown fibers on the forming surface. The forming surface typically completes at least one rotation about the longitudinal axis in the time required to advance the forming surface along the length of the meltblown fiber source. Where the forming surface completes two or more rotations in the time required to advance the forming surface along the length of the collector, a multilayer tubular web is built-up on the forming surface.
Because the forming surface rotates about the longitudinal axis while simultaneously advancing parallel to the longitudinal axis, the feathered edges in each layer of meltblown fibers are formed in a helical pattern on the cylindrical forming surface.
A separator is used to separate the tubular meltblown fibrous web in a direction oriented at an oblique angle relative to the longitudinal axis of the cylindrical forming surface. That oblique angle is equal to the helix angle formed by the feathered edges during manufacture of the meltblown fibrous web. The flat meltblown fibrous web formed after separating the tubular multilayer meltblown fibrous web along the helix angle includes two separated edges that have a thickness substantially the same as the thickness of the remainder of the web and does not require trimming or further processing before use of the web in other processes.
By separating the tubular multilayer meltblown fibrous web in a direction generally parallel to the helix angle to produce a flat multilayer meltblown fibrous web, variations in the density or weight of the meltblown fibrous web caused by the feathered edges are parallel to the edges of the formed flat web. This is in direct contrast with known tubular meltdown fibrous webs that are slit longitudinally, causing the feathered edges to cross the web at a bias angle with respect to the flat web centerline.
The meltblown fibrous webs of the present invention are different from known webs because of their multilayer composition in which the feathered edges are incorporated into the web and are disposed generally parallel to the web""s separated edges. Because the meltblown fibrous articles are commonly formed from multiple layers of meltblown fibers, variations in basis weight contributed by the feathered edges can be significantly reduced. The web layers that terminate in feathered edges form only a fraction of the overall basis weight of the web as a whole. In addition, any variations in the basis weight contributed by non-uniformities across the die or dies used to form the articles may also be reduced because of the multilayer nature of the articles. The helical nature of the process will naturally displace those variations over the width of the web such that they will not typically be aligned throughout the web thickness. Because the process causes the feathered edge to be included in the resulting web, the invention is advantageous in that it eliminates waste production stemming from the previous need to remove the feathered edge from the product.
These and other features and advantages of the meltblown fibrous webs, apparatus for manufacturing them, and methods for manufacturing them are discussed below in more detail.
In reference to the invention, the following terms are defined as set forth below:
xe2x80x9ccollectorxe2x80x9d means an apparatus that is capable of collecting meltblown fibers;
xe2x80x9cfeathered edgexe2x80x9d means the portion of a meltblown fibrous web layer in which the density and basis weight of the web tapers off due to fiber dispersal;
xe2x80x9cforming surfacexe2x80x9d means that portion of a collector on which meltblown fibers are deposited after exiting a meltblown fiber source;
xe2x80x9chelical patternxe2x80x9d means a pattern formed in the shape of helix, in other words, similar to the path followed by the threads of a screw;
xe2x80x9chelix anglexe2x80x9d means the angle formed by the helical pattern relative to a plane perpendicular to the longitudinal axis;
xe2x80x9cinterconnected layersxe2x80x9d means meltblown fiber layers that are connected to each other by, for example, fiber entanglement between fibers in the different layers, an agent introduced to connect the layers (for example, a resin, adhesive, etc.), and/or processing (for example, pin, welding, etc.);
xe2x80x9cintermediate layerxe2x80x9d means a meltdown fiber layer located between the first and second layers of a multilayer meltblown fibrous web;
xe2x80x9clongitudinal axisxe2x80x9d means the central axis about which the cylindrical forming surface rotates;
xe2x80x9cmachine directionxe2x80x9d means the direction of travel of the forming surface during formation of a tubular meltblown fibrous web;
xe2x80x9cmeltblown fiber layerxe2x80x9d and its variations means a nonwoven fibrous structure that contains meltblown fibers and possibly other ingredients formed on a collector or other surface during one pass past a source of meltblown fibers;
xe2x80x9cmeltblown fibrous webxe2x80x9d means a nonwoven fibrous structure that contains meltblown fibers and possibly other ingredients and that has sufficient integrity to be handlable by itself as a mat; and
xe2x80x9cseparated edgexe2x80x9d means an edge of a multilayer meltblown fibrous web that is physically separated from another edge of the web by any suitable method, for example, slitting, tearing, etc.